In conventional optical networks, the access network between an accommodating station and a subscriber terminal includes the following dedicated and shared types.
(1) Dedicated
The accommodating station and the subscriber terminal are interconnected by a dedicated optical cable.
(2) Shared
The accommodating station is connected to a branch point by an exclusive optical cable, and the branch point is connected to a plurality of subscriber terminals by respective optical cables. As shown below, active devices are used in some cases and passive devices are used in others.
2a) AON (Active Optical Network)
An active device is used at the branch point to differentiate the destination subscriber terminal based on the transmitted data.
2b) PON (Passive Optical Network)
At the branch point, a passive device called a splitter is used to simply separate the transmitted light. The following are some specific examples.
2b1) A-PON
Uses ATM (Asynchronous Transfer Mode) as the protocol.
2b2) B-PON
Uses Wavelength Division Multiplexing (WDM)
2b3) E-PON
Uses Ethernet (registered trademark) as the protocol.
Although among the above examples the dedicated access network is most desirable for the subscriber in terms of optimum degree of freedom and security, shared access networks are also widely used because the optical fiber facilities can be installed on a smaller scale in comparison. Further, among the shared access networks, the PON system is more frequently used because the distributed installation of the active devices in the AON system increases maintenance and management work.
In the PON (Passive Optical Network) system, the optical transmission paths are physically shared at some locations between the accommodating station and a plurality of the subscriber terminals. Access control is therefore indispensable at the shared locations, so that use of the optical transmission path is more restricted than in a dedicated system, owing to the need for, inter alia, standardization for the rules to share optical transmission path. Another disadvantage is that the splitter branching characteristics cause greater splitting loss with increasing number of branches. In addition, the splitter does nothing more than split, which means that downlink optical signals are broadcasted to all subscribers under the same splitter. This leaves room for security improvement from the viewpoint of the desirable physical separation of distributed signals. An additional problem is the difficulty of logical isolation in distribution to subscribers under the splitter and of monitoring the lines to the individual subscribers. Further, as regards resources of an access system, specifically a communication capacity at bottlenecks, a rigid passive system makes it difficult to respond flexibly to demand and supply with an eye to future increases in the number of subscribers.
The optical access system of the present invention performs optical path switching in the optical network, while Document 1 describes an optical node and optical branch insertion apparatus that can switch light pass of the optical network and can switch connection between a plurality of networks with an optical signal maintained as it is. For this, an intersection point of ring networks A, B each comprising at least two optical transmission paths is equipped with an optical node for optically interconnecting the plurality of ring networks A, B, which optical node comprises an optical cross-connect switch unit for switching and optically connecting a plurality of lines between the plurality of ring networks A, B and control means for controlling the switching of the optical cross-connect switch unit.
Moreover, the optical access system of the present invention is also a wavelength division multiplexing network, while a wavelength division multiplexing network is disclosed, for example, in Document 2. This is a wavelength division multiplexing network capable of performing high-capacity optical access service with a simple configuration. It has the following features. Specifically, in an optical network having a multi-echelon network, the highest-level network is a ring network having a center node and a remote node, the intermediate-level network has a ring configuration whose center node is a node belonging to the higher-order network, and the lowest-level network is centered on an accommodating station that aggregates traffic from a plurality of ONUs (optical network units) and is a star network directly connecting the accommodating station and each ONU by optical fibers, the center node belonging to the highest-level network and the ONUs use their respective lights with different wavelength to establish direct communication paths, and, at a node interposed between the two, amplification and branching, or routing are performed on the optical signal as it is without electrical processing.    Patent Document 1: Unexamined Japanese Patent Publication 2006-191212    Patent Document 2: Unexamined Japanese Patent Publication 2001-313660